As most of our foods are come from plant and/or animal origin, which are to be consider those characteristics of plant and animal tissues that may be affect by the growth of microorganisms. The plants such vegetables and other edible plant and animals which serve as food sources for us have all evolved mechanisms of defense against the invasion and proliferation of many microorganisms, and many of these remain in effect in fresh foods. By taking those natural food one can may be effective to use of each or all in preventing or retarding the growth of pathogenic and spoilage organisms in the products that are derived from them.
INTRINSIC PARAMETER
As most of our foods are come from plant and/or animal origin, which are to be consider those characteristics of plant and animal tissues that may be affect by the growth of microorganisms. The plants such vegetables and other edible plant and animals which serve as food sources for us have all evolved mechanisms of defense against the invasion and proliferation of many microorganisms, and many of these remain in effect in fresh foods. By taking those natural food one can may be effective to use of each or all in preventing or retarding the growth of pathogenic and spoilage organisms in the products that are derived from them.
INTRINSIC PARAMETER
The parameters which are an inherent part of the an organisms are referred to as intrinsic parameter.The parameters present in substrates in which the microorganisms are growing, that are internal parts of the substrate are called as intrinsic parameters.
These parameters are as follows:
1. pH
2. Moisture content
3. Oxidation–reduction potential (Eh)
4. Nutrient content
5. Antimicrobial constituents
6. Biological structures
1. pH:
All the microorganisms have a minimal, maximal and optimal pH for their growth,development, survival and activity of their enzymes. Growth of all may be microorganisms is affected by the pH of growth environments in food (growth medium) resulting large number of enzymes responsible for metabolism and growth. The Influenced of pH of any food not only has effect on their growth of microorganisms but also effect on their processing conditions. Food having acidic and basic contents promotes growth of acid loving microorganisms such as yeasts, moulds and some acidophilic bacteria.
Mold are very small in size which can grow over a wider range of acidic pH than bacteria and yeast. Most of the argumentative yeasts can easily grow at pH of about 4.0 to 4.5, in fruit juices and acid food such as sauerkraut and pickles. A food with an acidic pH would tend to be more micro-biologically stable than neutral or alkaline food. Because of this restrictive level pH of food such as fruits, soft drinks, fermented milks, sauerkraut and pickles are very stable against bacterial spoilage..
INTRINSIC PARAMETER
The parameters which are an inherent part of the an organisms are referred to as intrinsic parameter.The parameters present in substrates in which the microorganisms are growing, that are internal parts of the substrate are called as intrinsic parameters.
These parameters are as follows:
1. pH
2. Moisture content
3. Oxidation–reduction potential (Eh)
4. Nutrient content
5. Antimicrobial constituents
6. Biological structures
1. pH:
All the microorganisms have a minimal, maximal and optimal pH for their growth,development, survival and activity of their enzymes. Growth of all may be microorganisms is affected by the pH of growth environments in food (growth medium) resulting large number of enzymes responsible for metabolism and growth. The Influenced of pH of any food not only has effect on their growth of microorganisms but also effect on their processing conditions. Food having acidic and basic contents promotes growth of acid loving microorganisms such as yeasts, moulds and some acidophilic bacteria.
Mold are very small in size which can grow over a wider range of acidic pH than bacteria and yeast. Most of the argumentative yeasts can easily grow at pH of about 4.0 to 4.5, in fruit juices and acid food such as sauerkraut and pickles. A food with an acidic pH would tend to be more micro-biologically stable than neutral or alkaline food. Because of this restrictive level pH of food such as fruits, soft drinks, fermented milks, sauerkraut and pickles are very stable against bacterial spoilage..
Most of the bacteria, except acid fermenters are favored alkaline
or neutral pH. Most of the bacteria preferred a pH range between 7.0-7.5 but
some proteolytic bacteria can grow on food substrate with high pH. The buffer
content in the food is important to maintain the stability against microbial
spoilage.
Buffers permit an acid (or alkali) fermentation to go on longer
with a greater yield of products and organisms. Vegetable juices have low
buffering capacity permitting a decrease in pH with the production of only
small amount of acid by the lactic acid bacteria during the early stage of
sauerkraut and pickle fermentation. This helps to inhibit the growth of pectin
hydrolyzing and proteolytic competing bacteria in food.
Food acidification by fermentation in home food preparations is
the oldest practice man has been doing. It is due to production of organic
acids in food by growth and fermentation of microorganisms such as lactic and
acetic acid bacteria. The inhibitory properties of many of the organic acids
such as citric acid, lactic acid, benzoic acid, propionic acid, sorbic acids,
etc. can be used as effective acidulants or chemical preservatives against food spoilage bacteria.
2.Moisture
Content
One of the oldest methods of preserving foods is drying or
desiccation; precisely how this method came to be used is not known. The
preservation of foods by drying is a direct consequence of removal or binding
of moisture, without which microorganisms do not grow. It is now generally
accepted that the water requirements of microorganisms should be described in
terms of the water activity (aw) in the environment. This parameter is
defined by the ratio of the water vapor pressure of food substrate to the vapor
pressure of pure water at the same temperature: aw = p/po, where p is the vapor pressure of the solution and po
is the vapor pressure of the solvent
(usually water). This concept is related to relative humidity (RH) in the
following way: RH = 100 × aw.13 Pure water has an aw of 1.00, a 22% NaCl solution (w/v) has
an aw of 0.86, and a saturated solution of
NaCl has an aw of 0.75 (Table 3–4). The water activity (aw) of most fresh foods is above 0.99. The minimum
values reported for the growth of some microorganisms in foods is presented in
Table 3–5 (see also Chapter 18). In general, bacteria require higher values of aw for growth than fungi, with Gram-negative
bacteria having higher requirements than Gram positives. Most spoilage bacteria
do not grow below aw = 0.91, whereas spoilage molds can grow as low
as 0.80. With respect to food-poisoning bacteria, Staphylococcus aureus can grow as low as 0.86, whereas Clostridium
botulinum does not grow
below 0.94. Just as yeasts and molds grow over a wider pH range than bacteria,
the same is true for aw. The lowest reported value for foodborne bacteria is 0.75 for
halophiles (literally, “salt-loving”), whereas xerophilic
3. Oxidation–reduction potential (Eh)
The reducing and oxidizing power of the food will influence the
type of organism and chemical changes produced in the food. The concentration
of oxygen in food, chemical composition and type of microorganisms associated
contribute to the oxidation-reduction (O-R) potential of food and affect growth
of microorganisms in them. The O-R potential of a food may be defined as the
ease with which the substrate loses or gains electrons.
The Redox potential of food is determined by
characters such as:
(a) Oxygen tension of atmosphere above the food,
(b) Access of atmosphere to the food,
(c) Resistance of food to the changes occurring and
(d) O-R state of materials present in food.
On the basis of the ability of microorganism to utilize oxygen,
organisms are classified as aerobic, anaerobic and facultative anaerobes.
Aerobes require free oxygen and anaerobes don’t prefer oxygen as it is toxic to
them, hence, it is grow in the absence of molecular oxygen. Facultative may
grow both aerobic and anaerobic conditions.
Generally fungi- mould and yeasts are aerobic. But bacteria are
variables of these aspects. Some are aerobic, some are anaerobics and others
are facultative anaerobes. If oxidation potential is high then aerobes will
grow better than anaerobes, but if conditions become more reduced then
anaerobes will be the predominant organisms.The O-R potential is written as Eh
and measured and expressed as millivolts (mV). If the substrate is highly
oxidized would have a positive Eh and substrate is reduced is a negative Eh.
Aerobic microorganisms such as bacilli, cocci, micrococci, pseudomonas,
acinetobacters require and grow at positive O-R potential and anaerobe such as
Clostridia and bacteriodes require negative O-R potential for their growth.
Most of the fresh plant and animal food have low redox potential
because of reducing substances present in them. Fresh vegetables and fruits
contain reducing substances such as ascorbic acid, reducing sugars and animal
tissues have sulfhydryl (-SH) and other reducing group compounds considered as
antioxidants.
Fresh vegetables, fruits and meat promote growth of aerobic
microorganisms in the surface regions because of positive redox potential.
However, the anaerobic microorganisms grow in inner parts of vegetables, fruits
and meat because of negative redox potential. Most of processed plant and
animal food gain positive redox potential therefore promote growth of aerobic organisms.
4.Nutrient Content
4.Nutrient Content
In order to grow and function normally, the microorganisms of
importance in foods require the
following:
1. water
2. source of energy
3. source of nitrogen
4. vitamins and related growth factors
5. minerals
With respect to the other four groups of substances, molds have
the lowest requirement,followed by Gram-negative bacteria, yeasts, and
Gram-positive bacteria.As sources of energy, foodborne microorganisms may
utilize sugars, alcohols, and amino acids.Some microorganisms are able to
utilize complex carbohydrates such as starches and cellulose as sources of
energy by first degrading these compounds to simple sugars. Fats are also used
by microorganisms as sources of energy, but these compounds are attacked by a
relatively small number of microbes in foods. The primary nitrogen sources
utilized by heterotrophic microorganisms are amino acids. A large number of
other nitrogenous compounds may serve this function for various types of
organisms.
Some microbes, for example, are able to utilize nucleotides and free
amino acids, whereas others are able to utilize peptides and proteins. In
general, simple compounds such as amino acids will be utilized by almost all
organisms before any attack is made on the more complex compounds such as high-molecular-weight
proteins. The same is true of polysaccharides and fats. Microorganisms may
require B vitamins in low quantities, and almost all natural foods have an abundant
quantity for those organisms that are unable to synthesize their essential
requirements. In general, Gram-positive bacteria are the least synthetic and
must therefore be supplied with one or more of these compounds before they will
grow. The Gram-negative bacteria and molds are able to synthesize most or all
of their requirements. Consequently, these two groups of organisms may be found
growing on foods low in B vitamins. Fruits tend to be lower in B vitamins than
meats, and this
Antimicrobial
Constituents
The stability of some foods against attack by microorganisms is
due to the presence of certain naturally occurring substances that possess and
express antimicrobial activity. Some plant species are known to contain
essential oils that possess antimicrobial activity. Among these are eugenol in
cloves, allicin in garlic, cinnamic aldehyde and eugenol in cinnamon, allyl isothiocyanate
in mustard, eugenol and thymol in sage, and carvacrol (isothymol) and thymol in
oregano.47 Cow’s milk contains several antimicrobial substances, including
lactoferrin (see below), conglutinin, and the lactoperoxidase system (see
below). Raw milk has been reported to contain a rotavirus inhibitor that can
inhibit up to 106 pfu (plaqueforming units)/ml. It is destroyed by pasteurization.
Milk casein as well as some free fatty acids have been shown to be
antimicrobial under certain conditions.
Eggs contain lysozyme, as does milk, and this enzyme, along with
conalbumin, provides fresh eggs with a fairly efficient antimicrobial system. The
hydroxycinnamic acid derivatives (p-coumaric,
ferulic, caffeic, and chlorogenic acids) found in fruits, vegetables, tea, molasses,
and other plant sources all show antibacterial and some antifungal activity.
Lactoferrin is an iron-binding glycoprotein that is inhibitory to a number of
foodborne bacteria and its use as a microbial blocking agent on beef carcasses.
Ovotransferrin appears to be the inhibitory substance in raw egg white that
inhibits Salmonella
enteritidis.
5.Biological
Structures
The natural covering of some foods provides excellent protection against
the entry and subsequent damage by spoilage organisms. In this category are
such structures as the testa of seeds, the outer covering of fruits, the shell
of nuts, the hide of animals, and the shells of eggs. In the case of nuts such as
pecans and walnuts, the shell or covering is sufficient to prevent the entry of
all organisms. Once cracked, of course, nutmeats are subject to spoilage by
molds. The outer shell and membranes of eggs, if intact, prevent the entry of
nearly all microorganisms when stored under the proper conditions of humidity
and temperature. Fruits and vegetables with damaged covering undergo spoilage
much faster than those not damaged. The skin covering of fish and meats such as
beef and pork prevents the contamination and spoilage of these foods, partly
because it tends to dry out faster than freshly cut surfaces. Taken together,
these six intrinsic parameters represent nature’s way of preserving plant and
animal tissues from microorganisms. By determining the extent to which each
exists in a given food, one can predict the general types of microorganisms
that are likely to grow and, consequently, the overall stability of this
particular food. Their determination may also aid one in determining age, and
possibly the handling history of a given food.
EXTRINSIC
PARAMETERS
The
extrinsic parameters of foods are not substrate dependent. They are those
properties of the storage environment that affect both the foods and their
microorganisms. Those of greatest importance to the welfare of foodborne
organisms are as follows:
1.
temperature of storage
2.
relative humidity of environment
3.
presence and concentration of gases
4.
presence and activities of other microorganisms
Temperature of
Storage
Microorganisms,
individually and as a group, grow over a very wide range of temperatures.
Therefore, it is well to consider at this point the temperature growth ranges
for organisms of importance in foods as an aid in selecting the proper
temperature for the storage of different types of foods. The lowest temperature
at which a microorganism has been reported to grow is −34◦C; the highest is somewhere in excess of 100◦C. It is customary to place microorganisms
into three groups based on their temperature requirements for growth. Those
organisms that grow well at or below 7◦C and have their optimum between 20◦C and 30◦C are referred
to as psychrotrophs
. Those that grow well between 20◦C and 45◦C with optima between 30◦C and 40◦C are referred
to as mesophiles, whereas those that grow well at and above 45◦C with optima between 55◦C and 65◦C are referred to
as
thermophiles
Relative
Humidity of Environment
The RH of the storage environment is important both from the
standpoint of aw within foods and the growth of microorganisms at the surfaces.
When the aw of a food is set at 0.60, it is important that this food be
stored under conditions of RH that do not allow the food to pick up moisture
from the air and thereby increase its own surface and subsurface aw to a point where microbial growth can occur.
When foods with low aw values
are placed in environments of high RH, the foods pick up moisture until
equilibrium has been established. Likewise, foods with a high aw lose moisture when placed in an environment of low RH. There is
a relationship between RH and temperature that should be borne in mind in
selecting proper storage environments for foods. In general, the higher the
temperature, the
lower the RH, and vice versa. Foods that undergo surface spoilage
from molds, yeasts, and certain bacteria should be stored under conditions of
low RH. Improperly wrapped meats such as whole chickens and beef cuts tend to
suffer
much surface spoilage in the refrigerator before deep spoilage
occurs, due to the generally high RH of the refrigerator and the fact that the
meat-spoilage biota is essentially aerobic in nature. Although it is possible
to lessen the chances of surface spoilage in certain foods by storing under low
conditions of RH, it should be remembered that the food itself will lose
moisture to the atmosphere under such
conditions and thereby become undesirable. In selecting the proper
environmental conditions of RH, consideration must be given to both the
possibility of surface growth and the desirable quality to be maintained in the
foods in question. By altering the gaseous atmosphere, it is possible to retard
surface spoilage without lowering the RH.
Presence and
Concentration of Gases in the Environment
Carbon dioxide (CO2) is the single most important atmospheric gas
that is used to control microorganisms in foods.It along with O2 are the two
most important gases in modified atmosphere packaged (MAP) foods, Ozone (O3) is
the other atmospheric gas that has antimicrobial properties, and it has been tried
over a number of decades as an agent to extend the shelf life of certain foods.
It has been shown to be effective against a variety of microorganisms,9 but
because it is a strong oxidizing agent, it should not be used on
high-lipid-content foods since it would cause an increase in rancidity. Ozone
was tested against Escherichia
coli 0157:H7 in culture media, and at 3 to 18
ppm the bacterium was destroyed in 20 to 50 minutes.10 The gas was administered
from an ozone generator and on tryptic soy agar, the D value for 18 ppm was 1.18 minutes, but in phosphate buffer, the D value was 3.18 minutes. To achieve a 99% inactivation of about
10,000 cysts of Giardia
lamblia per milliliter, the average concentration
time was found to be 0.17 and 0.53 mg-min/L at 25◦C and 5◦C, respectively.53
The protozoan was about three times more sensitive to O3 at 25◦C than at 5◦C. It is allowed in foods in Australia, France, and Japan; and in
1997 it was accorded GRAS (generally regarded as safe) status in the United
States for food use. Overall, O3 levels of 0.15 to 5.00 ppm in air have been
shown to inhibit the growth of some spoilage bacteria as well as yeasts
Presence and
Activities of Other Microorganisms
Some
foodborne organisms produce substances that are either inhibitory or lethal to
others; these include antibiotics, bacteriocins, hydrogen peroxide, and organic
acids. The bacteriocins and some antibiotic
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